This invention relates to a process for the diffusion of aluminum into a semiconductor.
A P-type conductivity region in a semiconductor substrate has commonly been formed by diffusing under heat an impurity element of the group IIIb in the periodic table, and boron and gallium have been principally employed for this purpose. However, from the viewpoint of the coefficient of diffusion of various impurity elements into a substrate of a semiconductor such as silicon, aluminum has a greatest coefficient of diffusion, and the use of aluminum as a dopant provides such advantages among others that the length of time required for the diffusion can be shortened, and a deep diffused layer having a low impurity concentration can be easily formed, compared with the use of boron or gallium as the dopant. With the recent tendency toward a larger power capacity and a higher reverse breakdown voltage in various semiconductor devices, there is an ever-increasing demand for the provision of a P-type diffused layer which is sufficiently deep and has a low impurity concentration. This is because the formation of a deep P-type diffused having a low impurity concentration in a semiconductor substrate provides such advantages among others that the carrier injection efficiency can be improved to reduce the on-state voltage, and the surface field strength can be reduced to improve the reverse breakdown voltage level. In order to obtain such a low-concentration deep P-type diffused layer, therefore, the importance of the diffusion of aluminum into a semiconductor substrate is increasing more and more.
However, the prior art aluminum diffusion process has been defective in that the lifetime of minority carriers in a semiconductor substrate having aluminum diffused therein tends to be shortened. This shortened lifetime of minority carriers will adversely affect the operating characteristics of the semiconductor device in that the reverse leakage current will increase and the forward voltage drop will also increase. It has therefore been difficult hitherto to apply the aluminum diffusion process to a semiconductor device which has a large power capacity and a high reverse breakdown voltage and for which an extended lifetime of minority carriers is demanded.